The present invention relates to a field effect transistor using a semi-insulative compound substrate and a manufacturing method thereof.
In general, a Schottky gate field effect transistor (to be referred to as a "MESFET" hereinafter) comprises a semi-insulative compound substrate, e.g., a gallium-arsenide (GaAs) substrate. An active layer of a predetermined conductivity type (e.g., an n type) is formed in a surface portion of the GaAs substrate. A gate electrode is formed on the GaAs substrate so as to form a Schottky gate barrier together with the active layer. High concentration active regions of a predetermined conductivity type (e.g., an n.sup.+ type) serving as source and drain electrodes are formed in the substrate so as to be self-aligned with the gate electrode. When such self-aligned source and drain electrodes are formed, the substrate is subjected to ion-implantation using the gate electrode as a mask.
In order to respond to demand for highly integrated IC chips, when a higher packing density of MESFETs on substrates is to be achieved, the MESFETs must be micropatterned. In accordance with the micropatterning of MESFETs, the gate width and the distance between the source and drain regions is decreased. As a result, since an electric field applied to an active layer serving as a channel region is increased, a current flowing in the semi-insulative substrate under the active layer is increased. Such an increase in current; decreases the threshold value of the MESFET, undesirably increases the drain conductance, and decreases the mutual conductance. Such drawbacks notably occur in a MESFET using a semi-insulative substrate described above since the MESFET originally has a low potential barrier between the source and drain regions unlike a normal MOSFET using a conductive substrate. Therefore, it is difficult to form MESFETs with excellent electrical characteristics by micropatterning using a conventional technique.